AP and TS EAMCET Medical Model Paper 3

Among the following reactions which reaction is considered as "most crucial step (A)" of calvin cycle and which reaction (B) is catalysed by highly efficient carboxylase enzyme.
I) PEP + HCO3− → OAA
II)RUBP + CO2 + H2O → 2 3-PGA
III) RUPB + O2 → Phosphoglycerate + phosphoglycolate
IV) Malicacid → PEP + CO2